The error message says that:
/v1/marketplaces/TEST-MP29J5STPtZVvnjAFndM0N62/bank_accounts/BA7bk9dLT3My3fhAmZejgrKq" does not resolve to card
You're doing a:
customer.add_card('/v1/marketplaces/TEST-MP29J5STPtZVvnjAFndM0N62/bank_accounts/BA7bk9dLT3My3fhAmZejgrKq')
You should do this instead:
customer.add_bank_account('/v1/marketplaces/TEST-MP29J5STPtZVvnjAFndM0N62/bank_accounts/BA7bk9dLT3My3fhAmZejgrKq')